Transduction of signals that regulate cell growth and differentiation is regulated in part by phosphorylation of various cellular proteins. Protein tyrosine kinases are enzymes that catalyze this process. Receptor protein tyrosine kinases are believed to direct cellular growth via ligand-stimulated tyrosine phosphorylation of intracellular substrates.
The ErbB family of single-spanning, receptor tyrosine kinases consists of four members: epidermal growth factor receptor (EGFR), ErbB2 (HER2/neu), ErbB3 (HER3) and ErbB4 (HER4). A number of ligands, all of which are different gene products, have been identified that bind and activate EGFR (reviewed in Groenen et al., 1994). In contrast, a single neuregulin gene encodes for a large number of protein isoforms that result from alternative splicing of mRNA transcripts (reviewed in (Lemke, G. (1996) mol. Cell. Neurosci. 7:247-262). ErbB3 (Carraway, K. L. et al. (1994) J. Biol. Chem. 269:14303-14306) or ErbB4 (Plowman, G. D. et al., (1993) Nature 366:473-475) can serve as receptors for the neuregulins. These receptors and ligands play key roles in normal cell growth and differentiation.
Growth factor receptor protein tyrosine kinases of the class I subfamily include the 170 kDa epidermal growth factor receptor (EGFR) encoded by the erbB1 gene. erbB1 has been causally implicated in human malignancy. In particular, increased expression of this gene has been observed in more aggressive carcinomas of the breast, bladder, lung and stomach (Modjtahedi, H. and Dean, C. (1994) Int. J. Oncol. 4:277-296).
The second member of the class I subfamily, p185neu, was originally identified as the product of the transforming gene from neuroblastomas of chemically treated rats. The neu gene (also called erbB2 and HER2) encodes a 185 kDa receptor protein tyrosine kinase. Amplification and/or overexpression of the human HER2 gene correlates with a poor prognosis in breast and ovarian cancers (Slamon, D. J. et al., Science 235:177-182 (1987); and Slamon et al., Science 244:707-712 (1989)). Overexpression of HER2 has been correlated with other carcinomas including carcinomas of the stomach, endometrium, salivary gland, lung, kidney, colon and bladder. Accordingly, Slamon et al in U.S. Pat. No. 4,968,603 describe and claim various diagnostic assays for determining HER2 gene amplification or expression in tumor cells. Slamon et al. discovered that the presence of multiple gene copies of HER2 oncogene in tumor cells indicates that the disease is more likely to spread beyond the primary tumor site, and that the disease may therefore require more aggressive treatment than might otherwise be indicated by other diagnostic factors. Slamon et al. conclude that the HER2 gene amplification test, together with the determination of lymph node status, provides greatly improved prognostic utility.
A further related gene, called erbB3 or HER3, has also been described. See U.S. Pat. No. 5,183,884; Kraus et al., Proc. Natl. Acad. Sci. USA 86:9193-9197 (1989); EP Pat Appln No 444,961A1; and Kraus et al., Proc. Natl. Acad. Sci. USA 90:2900-2904 (1993). Kraus et al (1989) discovered that markedly elevated levels of erbB3 mRNA were present in certain human mammary tumor cell lines indicating that erbB3, like erbB1 and erbB2, may play a role in human malignancies. Also, Kraus et al (1993) showed that EGF-dependent activation of the ErbB3 catalytic domain of a chimeric EGFR/ErbB3 receptor resulted in a proliferative response in transfected NIH-3T3 cells. Furthermore, these researchers demonstrated that some human mammary tumor cell lines display a significant elevation of steady-state ErbB3 tyrosine phosphorylation further indicating that this receptor may play a role in human malignancies. The role of erbB3 in cancer has been explored by others. It has been found to be overexpressed in breast (Lemoine et al., Br. J. Cancer 66:1116-1121 (1992)), gastrointestinal (Poller et al., J. Pathol. 168:275-280 (1992), Rajkumer et al., J. Pathol. 170:271-278 (1993), and Sanidas et al., Int. J. Cancer 54:935-940 (1993)), and pancreatic cancers (Lemoine et al., J. Pathol. 168:269-273 (1992), and Friess et al., Clinical Cancer Research 1:1413-1420 (1995)).
ErbB3 is unique among the ErbB receptor family in that it possesses little or no intrinsic tyrosine kinase activity (Guy et al., Proc. Natl. Acad. Sci. USA 91:8132-8136 (1994) and Kim et al. J. Biol. Chem. 269:24747-55 (1994)). When ErbB3 is co-expressed with ErbB2, an active signaling complex is formed and antibodies directed against ErbB2 are capable of disrupting this complex (Sliwkowski et al., J. Biol. Chem., 269(20):14661-14665 (1994)). Additionally, the affinity of ErbB3 for heregulin (HRG) is increased to a higher affinity state when co-expressed with ErbB2. See also, Levi et al., Journal of Neuroscience 15: 1329-1340 (1995); Morrissey et al., Proc. Natl. Acad. Sci. USA 92: 1431-1435 (1995); Lewis, G. D. et al., Cancer Res., 56:1457-1465 (1996); Pinkas-Kramarski, R. et al. (1996) EMBO J. 15:2452-2467; Beerli, R. et al. (1995) Mol. Cell. Biol. 15:6496-6505; and Karunagaran, D. et al. (1996) EMBO J. 15:254-264 with respect to the in vivo ErbB2-ErbB3 protein complex.
The class I subfamily of growth factor receptor protein tyrosine kinases has been further extended to include the HER4/Erb4 receptor. See EP Pat Appln No 599,274; Plowman et al., Proc. Natl. Acad. Sci. USA 90:1746-1750 (1993); and Plowman et al., Nature 366:473-475 (1993). Plowman et al. found that increased HER4 expression closely correlated with certain carcinomas of epithelial origin, including breast adenocarcinomas. Diagnostic methods for detection of human neoplastic conditions (especially breast cancers) which evaluate HER4 expression are described in EP Pat Appln No. 599,274.
The quest for the activator of the HER2 oncogene has lead to the discovery of a family of heregulin polypeptides. These proteins appear to result from alternate splicing of a single gene which was mapped to the short arm of human chromosome 8 by Lee, J. and Wood, W. I. (1993) Genomics 16:790-791).
Holmes et al. isolated and cloned a family of polypeptide activators for the HER2 receptor which they called heregulin-α (HRG-α), heregulin-β1 (HRG-β1), heregulin-β2 (HRG-β2), heregulin-β2-like (HRG-β2-like), and heregulin-β3 (HRG-β3). See Holmes, W. E. et al., Science 256:1205-1210 (1992); WO 92/20798; and U.S. Pat. No. 5,367,060. The 45 kDa polypeptide, HRG-α, was purified from the conditioned medium of the MDA-MB-231 human breast cancer cell line. These researchers demonstrated the ability of the purified heregulin polypeptides to activate tyrosine phosphorylation of the HER2 receptor in MCF7 breast tumor cells. Furthermore, the mitogenic activity of the heregulin polypeptides on SK-BR-3 cells (which express high levels of the HER2 receptor) was illustrated. Like other growth factors which belong to the EGF family, soluble HRG polypeptides appear to be derived from a membrane bound precursor (called pro-HRG) which is proteolytically processed to release the 45 kDa soluble form. These pro-HRGs lack a N-terminal signal peptide.
While heregulins are substantially identical in the first 213 amino acid residues, they are classified into two major types, α and β, based on two variant EGF-like domains which differ in their C-terminal portions. Nevertheless, these EGF-like domains are identical in the spacing of six cysteine residues contained therein. Based on an amino acid sequence comparison, Holmes et al. found that between the first and sixth cysteines in the EGF-like domain, HRGs were 45% similar to heparin-binding EGF-like growth factor (HB-EGF), 35% identical to amphiregulin (AR), 32% identical to TGF-α, and 27% identical to EGF.
The 44 kDa neu differentiation factor (NDF), which is the rat equivalent of human HRG, was first described by Peles et al., Cell, 69:205-216 (1992); and Wen et al., Cell, 69:559-572 (1992). Like the HRG polypeptides, NDF has an immunoglobulin (Ig) homology domain followed by an EGF-like domain and lacks a N-terminal signal peptide. Subsequently, Wen et al., Mol. Cell. Biol., 14(3):1909-1919 (1994) carried out “exhaustive cloning” to extend the family of NDFs. This work revealed six distinct fibroblastic pro-NDFs. Adopting the nomenclature of Holmes et al., the NDFs are classified as either α or β polypeptides based on the sequences of the EGF-like domains. Isoforms 1 to 4 are characterized on the basis of the variable membrane stretch (between the EGF-like domain and transmembrane domain). Also, isoforms a, b and c are described which have variable length cytoplasmic domains. These researchers conclude that different NDF isoforms are generated by alternative splicing and perform distinct tissue-specific functions. See also EP 505 148; WO 93/22424; and WO 94/28133 concerning NDF.
While the heregulin polypeptides were first identified based on their ability to activate the HER2 receptor (see Holmes et al., supra), it was discovered that certain ovarian cells expressing neu and neu-transfected fibroblasts did not bind or crosslink to NDF, nor did they respond to NDF to undergo tyrosine phosphorylation (Peles et al., EMBO J. 12:961-971 (1993)). This indicated another cellular component was necessary for conferring full heregulin responsiveness. Carraway et al. subsequently demonstrated that 125I-rHRGβ1177-244 bound to NIH-3T3 fibroblasts stably transfected with bovine erbB3 but not to non-transfected parental cells. Accordingly, they conclude that ErbB3 is a receptor for HRG and mediates phosphorylation of intrinsic tyrosine residues as well as phosphorylation of ErbB2 receptor in cells which express both receptors. Carraway et al., J. Biol. Chem. 269(19):14303-14306(1994). Sliwkowski et al., J. Biol. Chem. 269(20):14661-14665 (1994) found that cells transfected with HER3 alone show low affinities for heregulin, whereas cells transfected with both HER2 and HER3 show higher affinities.
This observation correlates with the “receptor cross-talking” described previously by Kokai et al., Cell 58:287-292 (1989); Stern et al., EMBO J. 7:995-1001 (1988); and King et al., 4:13-18 (1989). These researchers found that binding of EGF to the EGFR resulted in activation of the EGFR kinase domain and cross-phosphorylation of p185HER2. This is believed to be a result of ligand-induced receptor heterodimerization and the concomitant cross-phosphorylation of the receptors within the heterodimer (Wada et al., Cell 61:1339-1347 (1990)).
Plowman and his colleagues have similarly studied p180HER4/p185HER2 activation. They expressed p185HER2 alone, p180HER4 alone, or the two receptors together in human T lymphocytes and demonstrated that heregulin is capable of stimulating tyrosine phosphorylation of p180HER4, but could only stimulate p185HER2 phosphorylation in cells expressing both receptors. Plowman et al., Nature 336:473-475 (1993). Thus, heregulin is an example of a member of the EGF growth factor family that can interact with several receptors (Carraway and Cantley, Cell 78:5-8 (1994)). Additionally, the β-cellulin ligand has been shown to bind to the EGF receptor and HER4, but does not bind HER3 (Riese II, D. J. et al. (1996) Oncogene 12:345-353).
The biological role of heregulin has been investigated by several groups. For example, Falls et al., (Cell 72:801-815 (1993)) found that ARIA plays a role in myotube differentiation, namely affecting the synthesis and concentration of neurotransmitter receptors in the postsynaptic muscle cells of motor neurons. Corfas and Fischbach demonstrated that ARIA also increases the number of sodium channels in chick muscle. Corfas and Fischbach, J. Neuroscience, 13(5): 2118-2125 (1993). It has also been shown that GGFII is mitogenic for subconfluent quiescent human myoblasts and that differentiation of clonal human myoblasts in the continuous presence of GGFII results in greater numbers of myotubes after six days of differentiation (Sklar et al., J. Cell Biochem, Abst. W462, 18D, 540 (1994)). See also WO 94/26298 published Nov. 24, 1994.
Holmes et al., supra, found that HRG exerted a mitogenic effect on mammary cell lines (such as SK-BR-3 and MCF-7). The mitogenic activity of GGFs on Schwann cells has also been reported. See, e.g., Brockes et al., J. Biol. Chem. 255(18):8374-8377 (1980); Lemke and Brockes, J. Neurosci. 4:75-83 (1984); Brockes et al., J. Neuroscience 4(1):75-83 (1984); Brockes et al., Ann. Neurol. 20(3):317-322 (1986); Brockes, J., Methods in Enzym., 147: 217-225 (1987) and Marchionni et al., supra. Schwann cells provide myelin sheathing around the axons of neurons, thereby forming individual nerve fibers. Thus, it is apparent that Schwann cells play an important role in the development, function and regeneration of peripheral nerves. The implications of this from a therapeutic standpoint have been addressed by Levi et al., J. Neuroscience 14(3): 1309-1319 (1994). Levi et al. discuss the potential for construction of a cellular prosthesis comprising human Schwann cells which could be transplanted into areas of damaged spinal cord. Methods for culturing Schwann cells ex vivo have been described. See WO 94/00140 and Li et al., J. Neuroscience 16(6):2012-2019 (1996).
Pinkas-Kramarski et al. found that NDF seems to be expressed in neurons and glial cells in embryonic and adult rat brain and primary cultures of rat brain cells, and suggested that it may act as a survival and maturation factor for astrocytes (Pinkas-Kramarski et al., PNAS, USA 91:9387-9391 (1994)). Meyer and Birchmeier, PNAS, USA 91:1064-1068 (1994) analyzed expression of heregulin during mouse embryogenesis and in the perinatal animal using in situ hybridization and RNase protection experiments. These authors conclude that, based on expression of this molecule, heregulin plays a role in vivo as a mesenchymal and neuronal factor. Also, their findings imply that heregulin functions in the development of epithelia. Similarly, Danilenko et al., Abstract 3101, FASEB 8(4-5):A535 (1994), found that the interaction of NDF and the HER2 receptor is important in directing epidermal migration and differentiation during wound repair.
Interaction of ErbB family members has been investigated in vitro and in vivo. Transactivation of ErbB2 as a result of ligand interaction with other ErbB family members is a common and physiologically important occurrence (Dougall, W. C. et al., (1993) J. Cell. Biochem. 53:61-73; Earp, H. S. et al, (1995) Breast Cancer Res. Treatment 35:115-132). Co-expression of ErbB2 with ErbB3 leads to the formation of a high affinity heregulin (HRG) binding site (Sliwkowski, M. X. et al., (1994) J. Biol. Chem. 269:14661-14665). ErbB2 modulates the affinity of ErbB3 for HRG and appears to provide tyrosine kinase activity to the ErbB3-HRG complex, since ErbB3 is a dysfunctional signaling receptor lacking intrinsic tyrosine kinase activity (Guy, P. M. et al., (1994) PNAS USA 91:8132-8136). Physio-chemical studies have not shown association of the ECDs of ErbB2 and ErbB3 in vitro (Horan et al., J. Biol. Chem. 270:24604-24608 (1995)). In addition, binding of neu differentiation factor (NDF) to soluble HER3 was not enhanced by the presence of soluble HER2.